Membrane separation process

ABSTRACT

Membrane separation process is improved by use, as a support membrane, of (i) a polyacrylonitrile membrane on which at least a portion of the surface --CN groups have been hydrolyzed --COOH groups or (ii) a membrane formed of polyacrylonitrile and polyacrylic acid.

RELATED APPLICATION

Application Serial Number 07/425,156 filed Oct. 23, 1989 by Texaco Inc.as assignee of Craig R. Bartels, now U.S. Pat. No. 4,992,176 issued Feb.12, 1991.

FIELD OF THE INVENTION

This invention relates to a membrane separation process. Moreparticularly it relates to a membrane system characterized by itsimproved life when used to separate charge systems typified by aqueousmixtures of ethylene glycol or isopropanol.

BACKGROUND OF THE INVENTION

As is well known to those skilled in the art, it is possible to separatemixtures of liquids, typified by mixtures of water and organic liquidssuch as aqueous solutions of ethylene glycol or isopropanol, by varioustechniques including adsorption or distillation. These conventionalprocesses, particularly distillation, are however, characterized by highcapital cost. In the case of distillation for example, the processrequires expensive distillation towers, heaters, heat exchangers(reboilers, condensers, etc.), together with a substantial amount ofauxiliary equipment typified by pumps, collection vessels, vacuumgenerating equipment, etc.

Such operations are also characterized by high operating costsprincipally costs of heating and cooling--plus pumping, etc.

Furthermore the properties of the materials being separated, as isevidenced by the distillation curves, may be such that a large number ofplates may be required, etc. When the material forms an azeotrope withwater, additional problems may be present which for example, wouldrequire that separation be effected in a series of steps (e.g. as in twotowers) or by addition of extraneous materials to the system.

There are also comparable problems which are unique to adsorptionsystems.

It has been found to be possible to utilize membrane systems to separatemixtures of miscible liquids by pervaporation. In this process, thecharge liquid is brought into contact with a membrane film; and onecomponent of the charge liquid preferentially permeates the membrane.The permeate is then removed as a vapor from the downstream side of thefilm--typically by sweeping with a carrier gas or by reducing thepressure below the saturated vapor pressure of the permeating species.

Illustrative membranes which have been employed in prior art techniquesinclude those set forth in the following table:

                  TABLE                                                           ______________________________________                                        Separating Layer   References                                                 ______________________________________                                        Nafion brand of    Cabasso and Liu                                            perfluorosulfonic acid                                                                           J. Memb. Sci. 24,                                                             101 (1985)                                                 Sulfonated polyethylene                                                                          Cabasso, Korngold                                                             & Liu J. Pol. Sc:                                                             Letters, 23, 57                                                               (1985)                                                     Fluorinated polyether                                                                            USP 4,526,948                                              or Carboxylic Acid fluorides                                                                     to Dupont as assignee                                                         of Resnickto                                               Selemion AMV       Wentzlaff                                                  brand of Asahi Glass                                                                             Boddeker & Hattanbach                                      cross-linked styrene                                                                             J. Memb. Sci. 22,333                                       butadiene (with quaternary                                                                       (1985)                                                     ammonium residues on a                                                        polyvinyl chloride backing)                                                   Cellulose triacetate                                                                             Wentzlaff, Boddeker                                                           & Hattanback, J. Memb.                                                        Sci. 22, 333 (1985)                                        Polyacrylonitrile  Neel, Aptel &                                                                 Clement Desalination                                                          53, 297 (1985)                                             Crosslinked        Eur. Patent 0 096                                          Polyvinyl Alcohol  339 to GFT as assignee                                                        of Bruschke                                                Poly(maleimide-    Yoshikawa et al                                            acrylonitrile)     J. Pol. Sci. 22, 2159                                                         (1984)                                                     Dextrine-          Chem. Econ. Eng.                                           isophorone diisocyanate                                                                          Rev., 17, 34 (1985)                                        ______________________________________                                    

The cost effectiveness of a membrane is determined by the selectivityand productivity. Of the membranes commercially available, anillustrative polyvinyl alcohol membrane of high performance is thatdisclosed in European patent 0 096 339 A2 of GFT as assignee ofBruschke--published Dec. 21, 1983.

European Patent 0 096 339 A2 to GFT as assignee of Bruschke discloses,as cross-linking agents, diacids (typified by maleic acid or fumaricacid); dihalogen compounds (typified by dichloroacetone or1,3-dichloroisopropanol); aldehydes, including dialdehydes, typified byformaldehyde. These membranes are said to be particularly effective fordehydration of aqueous solutions of ethanol or isopropanol.

This reference discloses separation of water from alcohols, ethers,ketones, aldehydes, or acids by use of composite membranes. Specificallythe composite includes (i) a backing typically about 120 microns inthickness, on which is positioned (ii) a microporous support layer of apolysulfone or a polyacrylonitrile of about 50 microns thickness, onwhich is positioned (iii) a separating layer of cross-linked polyvinylalcohol about 2 microns in thickness.

Polyvinyl alcohol may be cross-linked by use of difunctional agentswhich react with the hydroxyl group of the polyvinyl alcohol. Typicalcross-linking agent may include dialdehydes (which yield acetallinkages), diacids or diacid halides (which yield ester linkages),dihalogen compounds or epichlorhydrin (which yield ether linkages)olefinic aldehydes (which yield ether/acetal linkages), boric acid(which yields boric ester linkages), sulfonamidoaldehydes, etc.

U.S. Pat. No. 4,992,176 which issued Feb. 12, 1991 to Texaco as assigneeof Craig R. Bartels is directed to separation of water from organicoxygenates, such as isopropanol, by use of a membrane system including asupport layer of polyacrylonitrile bearing a separating layer ofpoly(vinyl pyridine) which has been cross-linked with an aliphaticpolyhalide.

U.S. Pat. No. 4,728,429 to Cabasso et al, U.S. Pat. No. 4,067,805 toChiang et al, U.S. Pat. No. 4,526,948 to Resnick, U.S. Pat. No.3,750,735 to Chiang et al, and U.S. Pat. No. 4,690,766 to Linder et alprovide additional background.

Additional prior art which may be of interest includes:

Mobility of Spin Probes in Quaternized Poly(4-Vinylpyridine) Membranes,Makino, Hamada, and Iijima, in Polym. J. (Toyko), 19(6), 737-45, 1987.

Effect of Quaternization on the Pervaporation Rate of Water ThroughPoly(4-Vinylpyridine) Membrane, Hamaya, and Yamada, in KobunshiRonbunshu, 34(7), 545-7, 1977.

Preparation of Separation Membranes, Yamamoto, Toi, and Mishima, patent#JP 61/161109 A2, Jul. 21 1986. (Japanese).

Separation of Some Aqueous Amine Solutions by Pervaporation throughPoly(4-Vinylpyridine) Membrane Yamada and Hamaya, in Kobunshi Ronbunshu,39(6), 407-14, 1982.

Complex Formation of Cross-linked Poly(4-Vinylpyridine) Resins withCopper (II), by Nishide, Deguchi, and Tsuchida, in Bulletin of theChemical Society of Japan, Vol. 49(12), 3498-3501 (1976).

Although many of these membrane systems of the prior art may exhibitsatisfactory Flux and Separation, it is found in practice that after themembrane assembly has been in use to effect a particular separation orfor an extended period of time, the assembly may tend to deteriorate andbecome brittle. In the membrane assembly of the above-noted U.S. Pat.No. 4,992,176 for example, it is found that mechanical stabilitydeteriorates to a degree that the Separation undesirable decreases.Although the length of time to reach this undesirable state with mayvary depending on the nature of the charge and the conditions ofoperation, it may occur in less than a few hours or in a few days.

Inspection of the membrane system reveals that deterioration is due tothe failure of the adhesion between the separating layer and the supportlayer. In the case for example of a polyvinyl pyridine separatingmembrane layer mounted on a polyacrylonitrile support, it is found thatthe bond therebetween has failed and this is evidenced by the visibleseparation of the layers as well as by the cracking of the separatingmembrane layer at those points at which the bond has failed.

It is an object of this invention to provide a membrane system,characterized inter alia by its ability to separate water from anorganic oxygenate typified by ethylene glycol, which possesses a highdegree of mechanical stability during such separation operations. Otherobjects will be apparent to those skilled in the art.

STATEMENT OF THE INVENTION

In accordance with certain of its aspects, this invention is directed toa membrane support layer, characterized by its high degree of bondingability to a membrane separating layer, comprising a membrane of acarbon-carbon backbone polymer containing --CN groups and a surfacethereof bearing --COOH groups.

DESCRIPTION OF THE INVENTION

The composite structure of this invention includes a multi-layerassembly which in the preferred embodiment preferably includes a porouscarrier layer which provides mechanical strength and support to theassembly.

THE CARRIER LAYER

This carrier layer, when used, is characterized by its high degree ofporosity and mechanical strength. It may be fibrous or non-fibrous,woven or non-woven. In the preferred embodiment, the carrier layer maybe a porous, flexible, non-woven or woven fibrous polyester.

One typical non-woven polyester carrier layer may be formulated ofnon-woven, thermally-bonded strands and characterized by a fabric weightof 80 ± 8 grams per square yard, a thickness of 4.2 ± 0.5 mils, atensile strength (in the machine direction) of 31 psi and (in crossdirection) of 10 psi, and a Frazier air permeability of 6 cuft/min/sq.ft. @ 0.5 inches of water.

THE POROUS SUPPORT LAYER

The porous support layer of this invention is preferably formed of amembrane of a carbon-carbon backbone polymer containing --CN groups anda surface thereof bearing pendant carboxyl (--COOH) groups. This layermay be formed of polyacrylonitrile, at least a portion of the surface--CN groups of which have been hydrolyzed to --COOH groups.Alternatively this layer may be formed by casting a mixture of apolyacrylonitrile and a polyacrylic acid.

When the porous support layer is formed from a polyacrylonitrile, thepolymer may typically be of molecular weight M_(n) of 5,000-100,000preferably 20,000-60,000, say 40,000. It may be the cast from a5w%-20w%, say 15w% solution thereof in inert solvent (typically asolvent such as dimethyl formamide, dimethyl sulfone, or dimethylacetamide) to form a layer of 40-80 microns, say 50 microns thick. Thecast membrane is then immersed in water at 4°-5° C. to 30° wherein thesolvent is extracted and the membrane sets up. Solvent such as that inwhich the polyacrylonitrile is dissolved, may also be added separatelyto the water bath to aid in membrane formation. The membrane is thenrinsed with water to remove solvent.

In practice of the process of this invention according to certain of itsaspects, the so-prepared porous support layer, which is formed of thecarbon-carbon backbone bearing --CN groups which characterizepolyacrylonitrile, is hydrolyzed to convert at least a portion of thesurface --CN groups to --COOH groups.

Although it may be possible to effect hydrolysis of thepolyacrylonitrile in solution prior to casting of the film or to attainthe desired results by use of a mixture (in solvent) of apolyacrylonitrile and a polyacrylic acid, it is preferred to prepare thedesired support by casting a polyacrylonitrile membrane and thentreating the surface to effect hydrolyis of --CN to --COOH.

Hydrolysis, in this embodiment, may be effected by contacting thesurface of the cured polyacrylonitrile support film with a hydrolyzingagent. Hydrolyzing agents commonly include aqueous solutions ofwater-soluble acids or bases, preferably Bronsted acids or bases inaqueous solution having a pH which is at least one unit distant from theneutral pH of 7 (i.e. below 6 or above 8). When the hydrolyzing agent isan acid, it is preferred that the pH of the agent be less than about 1.(Although acids of pH of 2-6 may be employed, it is found that thehydrolysis occurs slowly). When the agent is a base, the pH of the baseshould be above 8, say 9-13.

Typical acids which may be used to effect hydrolysis may include thefollowing strong (i.e. substantially completely ionized) acids:

                  TABLE                                                           ______________________________________                                                   sulfuric acid                                                                 hydrochloric acid                                                             nitric acid                                                                   hydrofluoric acid                                                  ______________________________________                                    

A typical strong acid which may be employed is a 16.6w% aqueous solutionof sulfuric acid which has a pH < 1.

Typical weak acids which may be used to effect hydrolysis may be thefollowing:

                  TABLE                                                           ______________________________________                                                  Acid                                                                ______________________________________                                                  formic acid                                                                   oxalic acid                                                                   acetic acid                                                                   propionic acid                                                                phosphoric acid (H.sub.3 PO.sub.4)                                  ______________________________________                                    

Typical bases which may be used to effect hydrolysis may include thefollowing strong (i.e. substantially completely ionized) bases:

                  TABLE                                                           ______________________________________                                                   sodium hydroxide                                                              potassium hydroxide                                                           sodium carbonate                                                              trisodium phosphate                                                ______________________________________                                    

A typical strong base which may be employed is a 10 w% aqueous solutionof sodium hydroxide.

Typical weak bases which may be used to effect hydrolysis may includethe following:

                  TABLE                                                           ______________________________________                                                  Base                                                                ______________________________________                                                  ammonium hydroxide                                                            pyridine                                                                      n-butyl amine                                                                 iso-butyl amine                                                               diethyl amine                                                                 diphenyl amine                                                                ethylene amine                                                      ______________________________________                                    

Hydrolysis may be effected by contacting the surface of theacrylonitrile porous support layer with the hydrolyzing agent at 70°C.-150° C., say 125° C. for 1-120, say 2 minutes. During this period, itmay typically be found that 1-100% mole %, typically 1-40 mole %, say 15mole % of the surface --CN groups may be hydrolyzed to --COOH groups.

In another embodiment, it may be possible to effect hydrolysis undersimilar conditions--prior to casting.

It should be noted that when hydrolysis is effected with a base, such assodium hydroxide, the --CN groups may be hydrolyzed to --COONa. It ispossible to convert this to the acid --COOH group by treatment withacid--but this is not necessary. In fact, slightly better results (interms of reproducibility, Separation, and Flux) may be attained with e.gthe --COONa salt-form than with the --COOH acid form.

In one of its embodiments the porous support layer may be formed from amixture of a polyacrylonitrile and a polyacrylic acid. This may becarried out by casting a 40-80 micron, say 50 micron layer from asolution in inert solvent containing (i) 60w%-99w%, say 85w% of apolyacrylonitrile of molecular weight M_(n) of 5,000-1000,000,preferably 20,000-60,000 say 40,000 and (ii) 1w%-40w%, say 15w% of apolyacrylic acid of molecular weight M_(n) of 90,000-300,000, preferably200,000-250,000 say 250,000.

The so-cast porous support layer may be cured at 125° C.-225° C., say150° C. for 1-30 minutes, say 10 minutes to yield a film having athickness of 40-80 microns, say 50 microns.

It appears likely that when the support layer is formed from the mixtureof polyacrylonitrile and polyacrylic acid and thereafter cured, aninterchange reaction may occur which yields a polymer which ischaracterized by a carbon-carbon backbone bearing both --CN and --COOHgroups which polymer is similar to that formed by partially hydrolyzinga polyacrylonitrile polymer.

It is a feature of these several embodiments that they are characterizedby the same mass properties of a polyacrylonitrile membrane (withrespect e.g. to separation ability etc.) while simultaneously possessingaugmented bonding properties in the membrane system because of themodified surface characteristics generated by the treatment of theinstant invention.

THE SEPARATING LAYER

There is then deposited on the 40-80 micron thick so-treated supportlayer, the separating layer. The separating layer may be any of a widerange of membranes depending on the charge to be separated and theconditions of separation. It might for example be (i) a cross-linkedpolyvinyl alcohol membrane (ii) a quaternary ammonium-exchangedfluorinated ion exchanged membrane, (iii) a sulfonated polypolyethylenemembrane, (iv) a silicone or silicone/polycarbonate membrane, (v) across-linked polyimine membrane, etc.

A preferred separating layer or membrane which permits attainment ofseparation in accordance with this invention includes a non-porous filmof cross-linked poly(vinyl pyridine) of thickness of about 1-10 microns,preferably 1-5 microns, say 3 microns. This layer is formed (preferablyby casting) from a poly(vinyl pyridine) solution. Although poly(2-vinylpyridine) may be employed, the preferred separating layer is preparedfrom poly(4-vinyl pyridine)--typically the Reilline 4200 brand (ofReilly Tar and Chemical Co) of poly(4-vinyl pyridine) in a 10 w%solution in a suitable alcohol solvent such as methanol.

The separating membrane may be formed by mixing 0.5-2 parts, say 1 partof the 10%-30%, say 10 w% solution of poly(4-vinyl pyridine) in methanolwith 1 part methanol, and 0.1-0.8 parts, say 0.52 parts of aliphaticpolyhalide cross-linking agent and casting the mixture on a support.

The separating layer may be a homopolymer or a copolymer of 2-vinylpyridine or more preferably 4-vinyl pyridine. When copolymers areemployed, the co-monomer may be an ethlenically unsaturated monomer,typically vinyl chloride, ethylene, vinyl alcohol, styrene, vinylacetate, ethylene oxide, etc. In the preferred embodiment, theseparating layer is a homopolymer of 4-vinyl pyridine of molecularweight M_(v) of 10,000-500,000, preferably 100,000-300,000, say about200,000.

The polymer may be cross-linked with a cross-linking agent to form themembranes useful in practice of this invention.

Typically the cross-linking agents may contain an aliphatic moiety,preferably containing 2-12 carbon atoms, typically 3-6 carbon atoms, say4 carbon atoms. Although the cross-linking agent may be a polyhalide, ittypically contains 2-5 halogen atoms, most preferably 2. The halogen ispreferably bromine or less preferably chlorine or iodine. The halidesmay preferably be alpha, omega dihalides of linear straight chainaliphatic hydrocarbon. Typical cross-linking agents may be as tabulatedinfra, the first listed being preferred:

                  TABLE                                                           ______________________________________                                                1,4-dibromo-n-butane (DBB)                                                    1,5-dibromo-n-pentane (DBP)                                                   1,3-dibromo propane                                                           1,6-dibromo hexane                                                            1,8-dibromo octane                                                            1,4-dichloro-n-butane                                                 ______________________________________                                    

In situ cross-linking may be carried out by casting onto the preferredtreated polyacrylonitrile support the poly(4-vinyl pyridine) typicallyin the solution in methanol to which has been added the cross-linkingagent (typically 1,4-dibromobutane) in mole ratio of cross-linking agentto polymer of 0.2-2, say about 1.13.

It may be possible in one embodiment to cross-link the poly(4-vinylpyridine) separating layer in one step by casting the solution ofpoly(4-vinyl pyridine) and polyhalide, followed by heat curing the castmembrane at 100° C.-200° C., say 125° C. for 1-30 minutes, say 2minutes.

In another embodiment, it may be possible to apply to the treated poroussupport layer, a solution of poly(4-vinyl pyridine). This may be driedat 40° C.-80° C., say 50° C. for 2-10 minutes, say 4 minutes to form afilm. There may then be added onto the surface of this uncross-linkedfilm a solution in methanol containing polyhalide and 2-7w%, say 3.5w%of poly(4-vinyl pyridine).

The composite membrane, whether prepared by the one-step or the two-stepprocess may then be cured in an oven at 100° C.-200° C., say 125° C. for1-30 minutes, say 2 minutes to yield a film having a thickness of 1-10microns, say 4 microns.

THE COMPOSITE MEMBRANE

It is a feature of this invention that the composite membrane maycomprise (i) an optional carrier layer, characterized by porosity andmechanical strength, for supporting a porous support layer and aseparating layer, (ii) as a porous support layer a membrane having acarbon-carbon backbone containing --CN groups and a surface thereofbearing --COOH groups, of molecular weight M_(n) of 5,000-100,000 ofthickness of 10-80 microns, and of molecular weight cut-off of25,000-100,000, and (iii) a non-porous separating layer, preferably ofpoly(vinyl pyridine) of molecular weight M_(v) of 10,000-500,000 whichhas been cross-linked with an aliphatic polyhalide.

The composite membranes of this invention may be utilized in variousconfigurations. It is, for example, possible to utilize the composite ina plate-and-frame configuration in which separating layers may bemounted on the porous support layer with the carrier layer.

It is possible to utilize a spiral wound module which includes anon-porous separating layer membrane mounted on a porous support layerand a carrier layer, the assembly being typically folded and bonded orsealed along all the edges but an open edge--to form a bag-like unitwhich preferably has the separating layer on the outside. A clothspacer, serving as the permeate or discharge channel is placed withinthe bag-like unit. The discharge channel projects from the open end ofthe unit.

There is then placed on one face of the bag-like unit, adjacent to theseparating layer, and coterminous therewith, a feed channelsheet--typically formed of a plastic net.

The so-formed assembly is wrapped around a preferably cylindricalconduit which bears a plurality of perforations in the wall--preferablyin a linear array which is as long as the width of the bag-like unit.The projecting portion of the discharge channel of the bag-like unit isplaced over the performations of the conduit; and the bag-like unit iswrapped around the conduit to form a spiral wound configuration.

It will be apparent that, although only one feed channel is present, thesingle feed channel in the wound assembly will be adjacent to two facesof the membrane layer. The spiral wound configuration may be formed bywrapping the assembly around the conduit a plurality of times to form areadily handleable unit. The unit is fitted within a shell(in mannercomparable to a shell-and-tube heat exchanger) provided with an inlet atone end and an outlet at the other. A baffle-like seal between the innersurface of the shell and the outer surface of the spiral-wound inputprevents fluid from bypassing the operative membrane system and insuresthat fluid enters the system principally at one end. The permeate passesfrom the feed channel, into contact with the separating layer and thencetherethrough, into the permeate channel and thence therealong to andthrough the perforations in the conduit through which it is withdrawn asnet permeate.

In use of the spiral wound membrane, charge liquid is permitted to passthrough the plastic net which serves as a feed channel and thence intocontact with the non-porous separating membranes. The liquid which doesnot pass through the membranes is withdrawn as retentate. The liquid orvapor which permeates the membrane passes into the volume occupied bythe permeate spacer and through this permeate channel to theperforations in the cylindrical conduit through which it is withdrawnfrom the system. In this embodiment, it will be apparent that the systemmay not include a carrier layer.

In another embodiment, it is possible to utilize the system of thisinvention as a tubular or hollow fibre. In this embodiment, the poroussupport layer of e.g. polyacrylonitrile may be extruded as a fine tubewith a wall thickness of typically 0.001-0.1mm. The extruded tubes arepassed through a aqueous bath of hydrolyzing agent, washed, and thenthrough a bath of e.g. poly(vinyl pyridine) in n-butanol which then iscross-linked and cured. A bundle of these tubes is secured (with anepoxy adhesive) at each end in a header; and the fibres are cut so thatthey are flush with the ends of the header. This tube bundle is mountedwithin a shell in a typical shell-and-tube assembly.

In operation, the charge liquid is admitted to the tube side and passesthrough the inside of the tubes and exits as retentate. During passagethrough the tubes, permeate passes through the non-porous separatinglayer and permeate is collected in the shell side.

In this embodiment, it will be apparent that the system may not normallyinclude a carrier layer.

PERVAPORATION

It is a feature of the membrane assembly including the membrane supportlayer and the non-porous separating layer mounted thereon that, althoughthis system may be useful in various membrane processes includingreverse osmosis, it is found to be particularly effective when used in apervaporation process. In pervaporation, a charge liquid containing amore permeable and a less permeable component is maintained in contactwith a non-porous separating layer; and a pressure drop is maintainedacross that layer. The charge liquid dissolves into the membrane anddiffuses therethrough. The permeate which passes through the membraneand exits as a vapor may be recovered by condensing at low temperatureor alternatively may be swept away by use of a moving stream of gas.Preferably, the permeate side of the membrane is maintained at a lowpressure, typically 5 mm. Hg.

For general background on pervaporation, note U.S. Pat. No. 4,277,344;U.S. Pat. No. 4,039,440; U.S. Pat. No. 3,926,798; U.S. Pat. No.3,950,247; U.S. Pat. No. 4,035,291; etc.

It is a feature of the process of this invention that the novel membranemay be particularly useful in pervaporation processes for dewateringaqueous mixtures of organic oxygenates. It may be possible to utilizethe process of this invention to remove water from immiscible mixturestherewith as in the case of ethyl acetate (solubility in water at 15° C.of 8.5 parts per 100 parts of water). It will be apparent to thoseskilled in the art that it may be desirable to separate large quantitiesof water from partially miscible systems as by decantation prior toutilizing the process of the invention to remove the last traces ofwater.

The advantages of the instant invention are more apparent when thecharge liquid is a single phase homogeneous aqueous solution as is thecase for example with aqueous solutions of isopropanol or ethyleneglycol. The system may also find use in the case of slightly solubleliquids wherein two phases are present (i) water-oxygenate first phaseand, as a second phase (ii) either water or oxygenate. Clearly thosecharge liquids which contain only a small portion of an immisciblesecond liquid phase may benefit most from the process of this invention.It is also a feature of this invention that it may be particularlyuseful to separate azeotropes such as isopropanol-water.

It is a particular feature of this invention that use of the membranesystem (preferably employing the treated--surface polyacrylonitrilesupport) permits attainment of separation systems which possess all theadvantages attained using untreated polyarylonitrile in addition tosubstantially improved mechanical and chemical stability.

The charge organic oxygenates which may be treated by the process ofthis invention may include alcohols, glycols, weak acids, ethers,esters, ketones, aldehydes, etc. It will be apparent to those skilled inthe art that the charge organic oxygenates used should be inert withrespect to the separating membrane. Clearly a system wherein themembrane is attacked by the components of the charge liquid will notyield significant separation for any reasonable period of time. Bestresults may be achieved when treating alcohols (such as isopropanol) orglycols (such as ethylene glycol). Results achieved with acids aregenerally less satisfactory.

Illustrative alcohols may include ethanol, propanol, i-propanol,n-butanol, i-butanol, t-butanol, amyl alcohols, hexyl alcohols, etc.

Illustrative glycols may include ethylene glycol, propylene glycols,butylene glycol or glycol ethers such as diethylene glycol, triethyleneglycol, or triols, including glycerine; etc.

Illustrative chlorinated hydrocarbons may include dichloroethane,methylene dichloride, etc.

Illustrative weak acids may include hexanoic acid, octanoic etc. (Whenacids are present, preferably the pH of the charge liquid should beabove about 4. Typical acids which may be treated by the process of thisinvention include those having a pKa ≧ ca 4.8.

Illustrative esters may include ethyl acetate, methyl acetate, butylacetate, methyl benzoate, ethylene glycol mono acetate, propylene glycolmonostearate, etc.

Illustrative ethers may include tetrahydroforan, diethyl ether,diisopropyl ether, etc.

Illustrative ketones may include acetone, methyl ethyl ketone,acetophenone, etc.

Illustrative aldehydes may include formaldehyde, acetaldehyde,propionaldehyde, etc.

It is believed that the advantages of this invention are most apparentwhere the organic oxygenate is a liquid which is infinitely misciblewith water--typified by isopropyl alcohol or ethylene glycol.

A typical charge may be an aqueous solution containing 70%-95%, say 85w%isopropanol.

In practice of the pervaporation process of this invention, the chargeaqueous organic oxygenate solution typically at 40° C.-120° C., say 80°C. may be passed into contact with the non-porous separating layer ofthe membrane of this invention. A pressure drop of about one atmosphereis commonly maintained across the membrane. Typically, the feed orcharge side of the membrane is at about atmospheric pressure and thepermeate or discharge side of the membrane is at a pressure of about2-50 preferably 5-20, say 10 mm. Hg.

The permeate which passes through the membrane includes water and asmall proportion of the organic oxygenate from the charge liquid.Typically, the permeate contains 70-99.5, say 75w% water. Permeate isrecovered in vapor phase.

Performance is judged by the ability of a membrane system to give apermeate containing decreased content of organic oxygenate (from acharge containing a higher content of organic oxygenate and water) witha good flux (kilograms-/meter² -/hour (kmh)) at a predetermined feedtemperature and with a vacuum on the permeate side and a condenser(cooled by liquid nitrogen). Compositions falling outside the scope ofthis invention may be characterized by unsatisfactory separation orunsatisfactory productivity (flux) or both.

Pervaporation may typically be carried out at a flux of about 0.5-2, say1.2 kilograms per square meter per hour (kmh). Typically, the units mayshow good separation (measured in terms of w% organic oxygenate in thepermeate during pervaporation of an aqueous solution of organicoxygenate through a poly(4-vinyl pyridine) separating layer.

It will be noted that as the concentration of the charge increases, theconcentration of oxygenate in the permeate increases and the Fluxdecreases.

Practice of the process of this invention will be apparent to thoseskilled in the art from inspection of the following examples wherein, aselsewhere in this specification, all parts are parts by weight unlessotherwise stated. An asterisk indicates a control example.

DESCRIPTION OF SPECIFIC EMBODIMENTS EXAMPLE I

In this Example which represents the best mode presently known ofcarrying out the process of this invention, the porous carrier layeremployed is the DUY-L brand of nonwoven polyester of the DaicelCorporation. The porous support layer is a microporous polyacrylonitrilemembrane layer of molecular weight M_(n) of 40,000. This layer istreated with an acid which is mild enough not to decrease the mechanicalstability yet strong enough to hydrolyze a portion of the acrylonitrileto the corresponding acrylic acid.

The acid hydrolyzing agent composition is prepared by mixing one part ofconcentrated sulfuric acid with five parts of water to yield a solutionof pH of < 1. This composition at 125° C. is placed in contact with thepolyacrylonitrile support layer for 2 minutes. During this time, 15 mole% of the surface --CN groups are converted to --COOH groups. This isevidenced by the broad peak at 1749-1688 cm⁻¹ in the infra red (FTIR),which corresponds to carboxylic acid stretching. The product containssurface --CN groups and surface --COOH groups.

This support is coated with poly(4-vinyl pyridine) by coating with asolution containing 2.5 parts of n-butanol, 2.5 parts of 20% poly(4vinyl pyridine) in methanol, and 1.3 parts of 1,4-dibromobutane. Thiscoating is dried at 125° C. for 2 minutes and cured at 125° C. for twominutes.

This membrane system is used to separate a charge liquid containing 85w%ethylene glycol and 15w% water. Charge is admitted to the pervaporationcell at 70° C. The permeate condenser yields an aqueous solutioncontaining 75w% water and only 25w% ethylene glycol. The Flux is 0.7 kmhand the permeate contains ca 75w% water.

After seven days of operation, the membrane was inspected. It exhibitedno evidence of separation, brittleness, or cracking. The surface was assmooth as the original after one week (168 hours) operation.

EXAMPLE II*

In this control Example, the procedure of Example I is duplicated exceptthat the polyacrylonitrile is not etched or treated with the sulfuricacid solution.

On disassembly and inspection after 3 hours of operation, it is foundthat the poly(vinyl pyridine) separating layer is brittle and cracked;and it has mechanically separated from the polyacrylonitrile supportlayer.

EXAMPLE III

In this Example, the procedure of Example I is carried out except thatthe polyacrylonitrile is etched, not with sulfuric acid, but with 40w%aqueous sodium hydroxide solution containing 10w% glycerine (to providea viscous, high boiling mixture). Analysis by FTIR showed a broad peakat 1651 cm⁻¹ corresponding to the carboxylic acid salt. (Rinsing thesurface with 10w% hydrochloric acid showed a shift of the 1651 cm⁻¹ peakto 1700 cm⁻¹ which is in the region of carboxylic acid stretching.)

The resulting membrane showed Flux of 1.6 kmh and no evidence of anymechanical separation after seven days. The permeate contains 80% water.

EXAMPLE IV

In this Example, the support layer is formed (as in Example I) from anaqueous solution containing 85w% polyacrylonitrile of M_(n) of 40,000and 15w% of polyacrylic acid M_(n) of 250,000. The resulting membranecontains Surface --CN and surface --COOH groups.

The procedure of Example I is otherwise followed to show and no evidenceof mechanical separation after seven days.

EXAMPLES V-IX

Results comparable to those of Example I may be attained if theseparating membrane layer is:

                  TABLE                                                           ______________________________________                                        EXAMPLE       Separating Membrane Layer                                       ______________________________________                                        V             Polyvinyl alcohol of -- M.sub.n of 115,000                                    cross-linked with glutaraldehyde -                                            pervaporation                                                   VI            Nafion-H 117 Fluorinated ion -                                                exchange membrane which has been                                              exchanged with tetra-n-octyl                                                  ammonium bromide - pervaporation                                VII           Sulfonated polyethylene which has                                             been contacted with LiCl -                                                    pervaporation                                                   VIII          Polyethylene imine which has been                                             cross-linked with toluene                                                     diisocyanate - reverse osmosis                                  IX            A blend of polyvinyl alcohol and                                              polyacrylic acid - pervaporation                                ______________________________________                                    

EXAMPLES X-XI-XII

This procedure of Example I is followed except that the polyvinylpyridine separating membrane is cross-linked with 1,6-dibromo hexaneinstead of an equal amount of 1,4-dibromobutane. In Control Example X*,the support membrane was neither acid nor base treated. In Example XI,the support membrane was etched with aqueous sulfuric acid and inExample XII, the support membrane was etched with 40% aqueous sodiumhydroxide.

In Control Example X*, the barrier cracked after 3-5 hours (Flux was 0.4kmh and Selectivity was 70%--water in permeate); but clearly since thesystem was leaking, this will drop to the same ratio as the feed. InExamples XI and XII (Flux 1.4 and 1.2 and Selectivity of 70-82% and80%), the separating film is as smooth as the original after one week ofoperation--with no evidence of separation of the membrane layers.

Although this invention has been illustrated by reference to specificembodiments, it will be apparent to those skilled in the art thatvarious charges and modifications may be made which clearly fall withinthe scope of the invention.

What is claimed:
 1. A membrane layer of a polyacrylonitrile, at least aportion of the surface --CN groups of which have been hydrolyzed to--COOH groups.
 2. A membrane support layer, characterized by it highdegree of bonding ability to a membrane separating layer comprising amembrane of a blend of a polyacrylonitrile and a polyacrylic acid.
 3. Amembrane system, characterized by its ability to separate water from acharge containing water and an organic oxygenate, which comprises amembrane support layer of a polyacrylonitrile, at least a portion of thesurface --CN groups of which have been hydrolyzed to --COOH groups; andmounted thereon a non-porous separating elastomer membrane layer.
 4. Themethod of treating a membrane layer of polyacrylonitrile which containssurface --CN groups which compriseshydrolyzing at least a portion ofsaid surface --CN groups to surface --COOH groups thereby forming amembrane layer containing surface --CN groups at least a portion ofwhich have been hydrolyzed to --COOH groups; and recovering saidmembrane layer containing surface --CN groups at least a portion ofwhich have been hydrolyzed to --COOH groups.
 5. The method of treating acharge containing water and organic oxygenate which comprisesmaintaininga membrane assembly including (i) a porous support layer of apolyacrylonitrile, at least a portion of the surface --CN groups ofwhich have been hydrolyzed to --COOH groups and (ii) a separatingelastomer membrane mounted on and bonded to said porous support layer;maintaining a pressure drop across said non-porous separating elastomermembrane; passing a charge aqueous solution of an organic oxygenate intocontact with the high pressure side of said non-porous separatingelastomer membrane whereby at least a portion of said water in saidcharge aqueous solution and a lesser portion of organic oxygenate insaid charge aqueous solution pass by pervaporation through saidnon-porous separating elastomer as a lean mixture containing more waterand less organic oxygenate than are present in said charge aqueoussolution and said charge aqueous solution is converted to a rich liquidcontaining less water and more organic oxygenate than are present insaid charge aqueous solution; recovering as permeate from the lowpressure side of said non-porous elastomer membrane, said lean mixturecontaining more water and less organic oxygenate than are present insaid charge aqueous solution, said lean mixture being recovered in vaporphase at a pressure below the vapor pressure thereof; and recovering asretentate from the high pressure side of said non-porous separatingmembrane said rich liquid containing a lower water content and a higherorganic oxygenate content than are present in said charge aqueoussolution.
 6. The method of treating a charge containing water andorganic oxygenate which comprisesmaintaining a membrane assemblyincluding (i) a porous support layer of a blend of a polyacrylonitrileand a polyacrylic acid and (ii) a separating elastomer membrane mountedon and bonded to said porous support layer; maintaining a pressure dropacross said non-porous separating elastomer membrane; passing a chargeaqueous solution of an organic oxygenate into contact with the highpressure side of said non-porous separating elastomer membrane wherebyat least a portion of said water in said charge aqueous solution and alesser portion of organic oxygenate in said charge aqueous solution passby pervaporation through said non-porous separating elastomer as a leanmixture containing more water and less organic oxygenate than arepresent in said charge aqueous solution and said charge aqueous solutionis converted to a rich liquid containing less water and more organicoxygenate than are present in said charge aqueous solution; recoveringas permeate from the low pressure side of said non-porous elastomermembrane, said lean mixture containing more water and less organicoxygenate than are present in said charge aqueous solution, said leanmixture being recovered in vapor phase at a pressure below the vaporpressure thereof; and recovering as retentate from the high pressureside of said non-porous separating membrane said rich liquid containinga lower water content and a higher organic oxygenate content than arepresent in said charge aqueous solution.